This invention relates to a method of measuring a substance level in a vessel by means of a level measuring device operating with bursts.
To determine level in a vessel, for example in a liquid tank or a bulk container, measuring devices operating with bursts are frequently employed which are commonly mounted above the substance whose level is to be measured.
One method of determining level, particularly quasi-continuously, is the pulse radar method. In this method, the level measuring device generates short, high-carrier-frequency microwave pulses or sound wave pulses, so-called bursts, at a repetition frequency in the range of a few hertz to a few megahertz, which are transmitted into a measured volume occupied by the substance, such as a filled volume of a liquid tank or a bulk container. If electromagnetic waves are used, a center-frequency range of the bursts preferably lies between about 0.5 and 24 GHz, while a center-frequency range of acoustic bursts lies between about 10 and 70 KHz.
Because of electrical or acoustic impedance discontinuities within the measured volume, the wave trains incident on and coupled into the contents of the vessel are partially reflected, particularly from the surface of the contents, back to the level measuring device and converted by the latter into a receive signal suitable for evaluation.
To couple the bursts into and out of the measured volume, antennas generating electromagnetic space waves and ultrasonic transducers generating electroacoustic space waves, respectively, are frequently used. Also known are level measuring devices which use a surface waveguide to conduct bursts as guided wave trains against the contents of the vessel. An essential measurand in the determination of level by the pulse radar method is a transit-time difference of the bursts, measured between a transit time of the transmit signal, which serves as a reference, and a transit time of the receive signal. Based on propagation velocities of transmitted waves and echo waves, which are known a priori or were determined, and on the geometry of the measured volume, which can be assumed to be known or be determined in operation, this transit-time difference can then be converted into a measured level value.
U.S. Pat. No. 6,087,978, for example, discloses a level measuring device operating with pulses and particularly with microwave pulses, which in operation couples transmitted waves into the vessel and receives echo waves reflected from contents of the vessel by means of a transducer element, which generates an envelope signal representative of amplitudes of transmitted waves and echo waves, and which derives from the envelope signal a measured value representative of the level of the contents.
Furthermore, the above-mentioned U.S. Pat. No. 6,087,978 as well as U.S. Pat. No. 4,521,778 and DE -A 44 07 369 disclose a corresponding method of measuring the transit time of microwave pulses using an envelope signal generated, for example, in the manner described above, the method comprising the steps of:
generating a digital sampling sequence from the envelope signal;
generating a first data tuple, which is representative of a reference point in said envelope signal;
generating a second data tuple, representative of an echo pulse in said envelope, by means of the sampling sequence, each of said data tupel having an amplitude value and an transit time value; and
deriving a measured level value from a difference between a transit-time value of the first data tuple and a transit-time value of the second data tuple.
The accuracy of the measured level value depends in particular on the accuracy with which the transit-time values of the transmit pulses and the useful echo can be determined. Because of the discrete sampling of the envelope signal, a maximum attainable accuracy of the measured level value is determined in prior-art level measuring devices by the resolution of the envelope signal, i.e., by the number of sample points in the sampling system referred to a unit of time or location of the envelope signal. This assumed maximum accuracy of the measured level value may be further reduced as a result of unavoidable signal noise, for example.
It is therefore an object of the invention to provide a level measuring method which provides an accurate measured value, particularly a value accurate within a millimeter, even with a coarse resolution of the envelope signal.
To attain this object, the invention provides a method of measuring level in a vessel by means of a level measuring device operating with pulses, the method comprising the steps of:
coupling waves to be transmitted into the vessel and receiving echo waves reflected from the contents of the vessel;
generating an envelope signal representative of transmitted waves and echo waves with at least one echo pulse;
generating a digital sampling sequence from the envelope signal;
generating a first data tuple, representative of a reference point in the envelope signal;
generating a second data tuple, representative of an echo pulse in the envelope signal, by means of the sampling sequence, each of the first and second data tuple having an amplitude value and a transit time value; and
deriving from the envelope signal a measured value representative of the level of the contents, this step comprising a step of determining a difference between the transit time value of the first data tuple and the transit time value of the second data tuple,
wherein the step of generating the second data tuple further comprises the steps of:
determining a first data record of the sampling sequence, the first data record being representative of a local extremum of the sampling sequence and having a transit time value greater than that of the first data tuple.
determining at least a second data record of the sampling sequence having a transit time value greater than that of the first data tuple and less than that of the first data record;
determining at least a third data record of the sampling sequence having a transit time value greater than that of the first data record;
deriving from the digital sampling sequence an extreme value representative of a highest amplitude value of the echo pulse by means of the first, second, and third data records; and
determining the transit time value of the second data tuple by means of the extreme value.
Furthermore, the invention consists in a method of measuring level in a vessel by means of a level measuring device operating with pulses, the method comprising the steps of:
coupling waves to be transmitted into the vessel and receiving echo waves reflected from the contents of the vessel;
generating an envelope signal representative of transmitted waves and echo waves with at least one echo pulse;
generating a digital sampling sequence from the envelope signal;
generating a first tuple, representative of a reference point in the envelope signal;
generating a second data tuple, representative of an echo pulse in the data tuple having an amplitude value and a transit time value; and
deriving from the envelope signal a measured value representative of the level of the contents, this step comprising a step of determining a difference between the transit time value of the first data tuple and the transit time value of the second data tuple;
wherein the step of generating the second data tuple further comprises the steps of:
deriving from the digital sampling sequence an extreme value representative of a highest amplitude value of the echo pulse;
determining the amplitude value of the second data tuple by means of a threshold value proportional to the extreme value; and
determining the transit time value of the second data tuple by means of the amplitude value of the second data tuple.
In a preferred embodiment of the method of the invention the waves to be transmitted are electromagnetic waves.
In a further preferred embodiment of the method of the invention the waves to be transmitted are microwaves.
In another preferred embodiment of the method of the invention the waves to be transmitted are sound waves.